In the design of lifting surfaces, e.g. wings of fixed wing aircraft, blades of rotary wing aircraft, blades of wind turbines, etc., a design point is generally chosen corresponding to a particular theoretical operating condition. The design point is often chosen as a mid-point of the operating regime. The shape of the lifting surface as manufactured is selected such that at the design point the lifting surface will deform to meet the anticipated shape at the design point.
One consequence of this design process is that at operating conditions other than the design point the shape of the lifting surface is likely to be aerodynamically sub-optimal. Moreover, even the theoretical operating condition corresponding to the design point may not be realised in practice.
Lifting surfaces are inherently flexible structures and a significant factor affecting the aerodynamic performance of the lifting surface is the tendency to twist and bend under aerodynamic loading. Local twist (local angle of attack) changes the local aerodynamic loading distribution, and this impacts the overall lifting surface lift, drag and moment characteristics. As an example, the lift induced drag of a lifting surface is dependent upon the spanwise lift distribution, which changes as the lifting surface twists and bends under load. In addition the necessary strength (and weight) of a lifting surface structure is dependent on the spanwise lift distribution—a lifting surface with a lift distribution more biased towards the wing tip will typically be heavier than one with a lift distribution more biased towards the lifting surface root, due to higher bending moments.
The flexural axis is defined as the line of shear centres of the aerofoil sections in the spanwise direction of the lifting surface, where the shear centre is the point in the section where a shear load causes no twist and a torque causes no bending.
Many fixed and rotary wings have an additional lifting surface, a so called “wing tip device”, or “winglet”, attached at the outboard end of the wing. These wing tip devices are typically designed to reduce lift induced drag. Wing tip devices may be fixed to the outboard end of the wing so as to be non-moveable, or may be moveable and actively controlled.
US 2006/0049307 describes a wing elastically deformable about an elastic axis and with an actively controlled surface at the wing tip for minimising the lift induced drag of the wing.